For high brightness applications, it is common to mount an array of LED dies on a substrate, where the substrate has metal traces that interconnect the LED dies and lead to anode and cathode electrodes for connection to a power supply. It is common for the LED dies to be GaN-based and emit blue light, where a phosphor (e.g., a YAG phosphor) is deposited over all the LED dies. The combination of the blue light leaking though the phosphor and the yellow-green phosphor light creates white light.
Some light from the LED dies and some light from the phosphor is emitted in the downward direction and is partially absorbed by the substrate. Further, if the individual LED dies are also mounted on submounts (typically much larger than the LED die), and the submount electrodes are bonded to the substrate, the submount surface also absorbs some of the LED and phosphor light. Such absorption by the substrate and submount reduces the overall efficiency of the module.
What is needed is a packaging technique for phosphor-converted LEDs that results in more light being emitted by the package.